Optical absorption spectroscopy based gas analyzer systems and methods

ABSTRACT

The present invention provides a system for measuring concentrations of trace gases in gas mixtures using the absorption spectroscopy method. The system comprising an optical cell containing a gas mixture, a continuous-wave tunable laser, a detector system for measuring an absorption of laser light by the gas in the optical cell, and a processor to conduct an absorption spectroscopy analysis of the gas mixture based on light intensity measured by the detector system at one or more laser frequencies.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 16/457,999, filed onJun. 29, 2019 and hereby incorporated by reference its entirety.Application U.S. Ser. No. 16/457,999 is a continuation of U.S. Ser. No.15/859,378 filed on Dec. 30, 2017 and hereby incorporated by referenceits entirety. Application U.S. Ser. No. 15/859,378 claims the benefitof, and priority to, U.S. provisional Patent application No. 62/535,505filed on Jul. 21, 2017 and hereby incorporated by reference itsentirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to trace gas detection and morespecifically to cavity enhanced absorption spectroscopy systems andmethods for measuring trace gases.

Traditionally, in standard applications of cavity enhanced absorptionspectroscopy methods with resonant cavities, lasers with smoothfrequency tuning behaviors are preferable. It is also important to havelasers with smooth frequency tuning behaviors for tunable diode laserabsorption spectroscopy applications, e.g, TDLAS or Off Axis ICOS, whereconcentrations of absorbing gas species are measured by measuringabsorption spectra of different species as function of wavelength andfitting them using spectral line-shape models. Unless a very precisewave-meter is used to measures the laser light frequency, any deviationsin the laser tuning curves from ideal may cause errors in the reportedconcentration values.

In addition to that, if a DFB laser is used in a cavity enhanced laserbased gas analyzer system as a light source, an electrical noise of thelaser diode current causes an additional noise in the laser frequency.Moreover, any unwanted discontinuity in the laser current tuning, forexample, due to the quantization noise of a finite resolution of adigital-to-analog converter, can be transferred to a discontinuity ofthe laser frequency.

Accordingly, there is a need for systems and methods for trace gasdetection using lasers with improved performance coupled to resonanceoptical cavities.

SUMMARY OF THE INVENTION

The present invention provides systems and methods for measuringconcentrations of trace gases in gas mixtures using absorptionspectroscopy methods.

Embodiments of the present invention provide systems and devices formeasuring concentrations of trace gases in analyzed gas mixtures withhigh accuracy using a resonance optical cavity, which contains a gasmixture to be analyzed, a laser coupled to the cavity, and a lightsensitive detector. The optical cavity can include any type of cavitywith two or more cavity mirrors, including a linear or a ring cavity. Alaser that is capable of being frequency-scanned is coupled to thecavity through one of the cavity mirrors (i.e., the cavity couplingmirror). A detection method can be based on any of a variety of cavityenhanced optical spectroscopy (CEOS) methods, for example, cavityring-down spectroscopy (CRDS) methods, or cavity enhanced absorptionspectroscopy (CEAS) methods.

Embodiments of the present invention also provide a system comprising alaser source with a stepwise tuning curve where frequency steps arematched to the free spectral range (FSR) of a resonant cavity. Anexample of such laser source is an external cavity laser based on a DFBlaser. At a specific range of the optical feedback created by theexternal cavity, the laser linewidth significantly decreases incomparison with a free running DFB laser and its spectral tuning curvebecomes a stepwise function.

Positions of steps in the laser tuning curve can be tuned by adjustingthe external cavity optical length. Such laser source can be fibercoupled to deliver the laser light to a resonant cavity by an opticalfiber. Such laser source can also be optically isolated from externaloptical feedback sources. Laser light emitted by such laser source couldalso be amplified, modulated or interrupted by a SOA or a BOA. Laserlight emitted by such laser source can also be modulated or interruptedby optical modulators, such as AOM, EOM, etc. When laser light emittedby such laser source is coupled to a resonant cavity, the couplingefficiency can be significantly increased because of two reasons: thelaser line width narrows and the laser frequency tuning slope decreaseson flattened parts of the stepwise tuning curve (the laser frequencybecomes less sensitive to the laser current driver noise) compared to afree running DFB laser. This invention is both an apparatus and a methodof using such laser source for the cavity enhanced optical absorptionspectroscopy applications with resonant cavities.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 illustrates a cavity enhanced absorption spectroscopy systemaccording to one embodiment.

FIG. 2 illustrates a cavity enhanced absorption spectroscopy systemaccording to another embodiment.

FIG. 3 illustrates a cavity enhanced absorption spectroscopy systemaccording to yet another embodiment.

FIG. 4 illustrates an optical absorption spectroscopy system using a gascell according to another embodiment.

FIG. 5 illustrates four typical laser tuning curves of an externalcavity laser, where a laser gain media is a DFB laser depending onintensity of the optical feedback sent to the gain media by the externalcavity.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides systems and methods for measuringconcentrations of trace gases in gas mixtures using absorptionspectroscopy methods. The optical absorption spectroscopy involvespassing radiation through a sample, e.g., an analyte and measuringabsorption property of the sample as a function of the radiationwavelength. For example, trace gas detection can be spectroscopicallyperformed by taking measurements to detect the presence or absence ofspectral absorption lines corresponding to the gas species of interest.Trace gas detection can be spectroscopically performed by takingmeasurements to quantify spectral absorption lines corresponding to thegas species of interest and to compute concentrations of analytes, gaspressure, and gas temperature. Spectroscopic analysis of isotopologuescan also be performed.

One of the most important applications of the optical absorptionspectroscopy technique is measuring gases in trace amounts.Concentrations of gases can be less than part-per-billion. The taskbecomes more challenging if relatively weak rotational-vibrationaltransitions in the NIR spectral range are measured for determining thetrace gas concentration. Such measurements usually require high finenessresonant cavities, which effectively increase the path length of aninteraction of laser light with trace gases. However, high finenessresonant optical cavities have rather narrow optical resonances, usuallymuch more narrow than linewidth of lasers used for such applications,such as DFB lasers. This phenomenon drastically decreases in efficiencyof the laser-to-cavity coupling. Making external cavity lasers based onDFB lasers as gain media helps to partially solve two problems:improving the laser-to-cavity coupling due to mere narrow linewidth ofexternal cavity lasers and decreasing sensitivity of wavelength of theemitted light of such lasers to the electrical noise.

We use a term optical path length, or optical length, or opticaldistance meaning the product of the geometric length of the path lightfollows through the system, and the index of refraction of the mediumthrough which it propagates.

According to an embodiment, a gas analyzer system for measuring aconcentration of a component in a gas mixture, the system comprising: aresonant optical cavity containing said gas and having at least twocavity mirrors, one of which is a cavity coupling mirror containing agas having a chemical species to be measured; a continuous-wave stepwisetunable external cavity laser emitting light optically coupled to theresonant optical cavity, an output of the laser is scanned across arange of frequencies including the frequency of said one of theplurality of cavity modes; mode matching optics configured to couple thelaser light to the cavity via the cavity coupling mirror; a detectorsystem for measuring an absorption of laser light by the gas in theresonant optical cavity, the detector system being operable to transmita data signal corresponding the absorption of laser light by the gas inthe resonant optical cavity; and a processor operable to conduct theabsorption spectroscopy analysis of the gas sample based on the datasignal. The roundtrip length of the cavity of the external cavity laseris close to the roundtrip length of the resonant optical cavitycontaining the measured gas mixture. When the optical feedback from theexternal cavity to the laser gain media is strong enough to produce astepwise laser tuning curve, but weak enough to not skip some modes ofthe external laser cavity, the laser produces a stepwise tuning curvewith a difference in frequencies of two adjusted steps closer to thefree spectral range of the resonant cavity.

According to an embodiment, an external cavity laser comprises a meansfor adjusting an optical length of the external cavity of the laser tomatch a frequency of the lasing light to a frequency of one of thecavity modes of the resonant optical cavity. Such adjustment can be doneby changing the physical length of the external cavity, for example byplacing one of mirrors of the cavity on a translation stage or on apiezo transducer. Such adjustment can also be done by changing theoptical length of one of the optical components of the external cavity,such as a phasor placed in the cavity, a lens, etc. The optical lengthof the optical path through a lens can be changed by changing the lenstemperature, which affects the refractive index of the lens material.Such adjustment can also be done by changing the gas pressure inside theexternal cavity.

According to another embodiment, a resonant cavity comprises a means foradjusting an optical length of the resonant cavity to match thefrequency of the lasing light to a frequency of one of the cavity modesof the resonant optical cavity. Such adjustment can be done by changingthe physical length of the resonant cavity, for example by placing oneof mirrors of the cavity on a piezo transducer. Such adjustment can alsobe done by changing the gas pressure inside the resonant cavity.

According to an embodiment, an external cavity laser comprises a meansfor adjusting an optical feedback intensity from one of mirrors formingthe laser external cavity to a laser gain media to match steps of thelaser tuning curve to the free spectral range of the resonant cavity. Ifthe feedback is too weak, no steps are produced in the laser tuningcurve. If the feedback is too strong, the steps can be wider than thefree spectral range of the external cavity. With intermediate feedbackintensity, the laser can be repeatedly tuned to any external cavitymode. The feedback intensity can be adjusted by placing any lightintensity modulator inside the laser cavity. The feedback intensity canalso be adjusted by tilting one of the mirrors of the external cavity orby misaligning another optical component in the laser cavity. FIG. 5illustrates how the laser tuning curve may depend on the opticalfeedback to the gain media in an external laser cavity, wherein themedia is a DFB laser. The feedback strength is increasing from FIG. 5Ato FIG. 5D. FIG. 5A illustrates the laser tuning curve of a free runningDFB laser with no feedback from any external to the DFB laser sources.

According to an embodiment, the external cavity laser comprises a singlemode optical fiber as part of the external cavity of the laser. Theoptical feedback to the gain media can be produced by a reflection fromone of the ends of the optical fiber. The optical feedback to the gainmedia can also be produced by a reflection from an optical component,which is a part of the external cavity. An example of such laser is afiber coupled DFB laser without any optical isolator between a gainmedia and an optical fiber. If an optical collimator is used tocollimate the laser beam emitted from the fiber, then a partialreflector can be placed at the collimated free space beam. When thepartial reflector is perpendicular to the laser beam it reflects back tothe fiber light emitted from the fiber. By tilting the partialreflector, the intensity of the optical feedback to the gain media canbe adjusted. The phase of the stepwise tuning curve can be adjusted bychanging the optical distance between the gain media and the partialreflector. The optical distance between the gain media and the partialreflector can be adjusted, for example, by changing a temperature of theoptical fiber, by changing a mechanical stress in the fiber, or by adistance between the fiber and the partial reflector.

According to another embodiment, the external cavity laser is opticallyisolated from the resonant cavity. It can be done by placing an opticalisolator in the optical path between the laser and the cavity. A Faradayisolator is an example of such optical isolator.

According to an embodiment, the gas analyzer system for measuring aconcentration of a component in a gas mixture comprising an opticalamplifier in an optical path between the continuous-wave stepwisetunable external cavity laser and the resonant cavity. The opticalamplifier selected from the group consisting of a Booster OpticalAmplifier (BOA) and a Semiconductor Optical Amplifier (SOA), the opticalamplifier is capable to amplify or/and to interrupt laser light emittedby the laser. The amplifier can be a free space amplifier or a fibercoupled amplifier. The laser can be optically isolated from theamplifier. The amplifier can also be optically isolated from theresonant cavity. The amplifier can be used to amplify the laser lightintensity. The amplifier can also be used to interrupt the laser light.For example, the amplifier can be used to interrupt the laser light ifthe gas analyzer is based on the CRDS technique.

According to an embodiment the gas analyzer system for measuring aconcentration of a component in a gas mixture comprising an opticalmodulator in an optical path between the continuous-wave stepwisetunable external cavity laser and the resonant cavity, wherein opticalmodulator selected from the group consisting of an Electro-OpticModulator (EOM) and an Acousto-Optic Modulator (AOM), the opticalmodulator is capable to modulate the laser light intensity emitted bythe laser. The laser can be optically isolated from the modulator. Themodulator can also be optically isolated from the resonant cavity. Themodulator can be used to interrupt the laser light. For example, themodulator can be used to interrupt the laser light if the gas analyzeris based on the CRDS technique.

According to another embodiment the detector system includes aphoto-detector configured to measure an intensity of intra-cavity light.The photo-detector can be a photodiode, a photoresistor, photovoltaicdetector, or another type of photo-detector.

According to an embodiment, the optical length between thephoto-detector and the cavity mirror used to transmit the light measuredby the photo-detector is close to half of the roundtrip optical lengthof the resonant cavity. This arrangement permits to minimize an effectof spectral ripples created by the light scattered or reflected from thedetector and coupled back to the resonant cavity.

According to another embodiment the optical length of the optical pathbetween one of the sources of unwanted scattered or reflected lightcoupled to the resonant cavity and the coupling mirror of the resonantcavity is close to half of the roundtrip optical length of the resonantcavity. Usually all optical components scatter light. Sources of thelight scattering can be defects in optical coatings, roughness ofsurfaces of optical components, or bulk defects in optical components.These defects created so called coupled cavities with the resonantcavity. By placing an optical component with strong scattering orreflections coupled back to the cavity at the specify distance one canminimize an effect of spectral ripples created by this opticalcomponent.

According to another embodiment, an optical absorption spectroscopybased gas analyzer system for measuring a concentration of a componentin a gas mixture, the system comprising: an optical cell containing agas having a chemical species to be measured; a continuous-wave stepwisetunable external cavity laser emitting light entering the cell, theexternal cavity laser having a semiconductor distributed feedback laser(DFB) as gain media, wherein an output of the DFB laser is scannedacross a range of the plurality of cavity modes of the external cavity;a detector system for measuring the intensity of the laser beam afterthe laser beam passes through the gas in the optical cell, the detectorsystem being operable to transmit a data signal corresponding to theintensity of the laser beam after the laser beam passes through the gasin the optical cell; and a processor operable to conduct the absorptionspectroscopy analysis of the gas sample based on the data signal;wherein steps in a laser tuning curve are used as frequency markers inan analysis of measured spectra.

According to yet another embodiment, the cavity of the external cavitylaser can be temperature controlled.

According to an embodiment the optical cell is a multipass spectroscopicabsorption optical cell.

FIG. 1 is a schematic diagram of a system 1 of the present invention. InFIG. 1, light beams are illustrated by solid lines. For clarity ofpresentation, various standard elements such as lenses and mirrors usedfor focusing and directing beams are not described; such elements arewell known in the art. Processor and electrical connections are notshown. System 1 comprises resonant optical cavity 2 of optical cavitylength 11 containing a gas and having at least two cavity mirrors, oneof which is cavity coupling mirror 5, another mirror 6 is used totransmit the light measured by the photo-detector. System 1 alsocomprises continuous-wave stepwise tunable external cavity laser 3 ofoptical cavity length 12 emitting light optically coupled to theresonant optical cavity 2, wherein the external cavity laser comprisessemiconductor distributed feedback laser 7 as gain media and outputcoupler 8 forming a laser cavity; mode matching optics 10 configured tocouple the laser light to resonant cavity 2 via cavity coupling mirror5. System 1 also comprises a detector system 4 for measuring theabsorption of laser light by the gas in resonant optical cavity 2,detector system 4 being operable to transmit a data signal correspondingthe absorption of laser light by the gas in resonant optical cavity 2,detector system 4 separated from mirror 6 by optical length 13.Continuous-wave stepwise tunable external cavity laser 3 is opticallyisolated from cavity 2 by optical isolator 9. Continuous-wave stepwisetunable external cavity laser 3 is fiber coupled to optical fiber 15.Mode matching optics 10, which can be a source of scattered lightcoupled back to cavity 2 is separated from coupling mirror 5 by opticallength 14.

FIG. 2 is another schematic diagram of system 1 of the presentinvention. In FIG. 2, light beams are illustrated by solid lines. Forclarity of presentation, various standard elements such as lenses andmirrors used for focusing and directing beams are not described; suchelements are well known in the art. Processor and electrical connectionsare not shown. System 1 comprises resonant optical cavity 2 of opticalcavity length 11 containing a gas and having at least two cavitymirrors, one of which is cavity coupling mirror 5, another mirror 6 isused to transmit the light measured by the photo-detector. System 1 alsocomprises continuous-wave stepwise tunable external cavity laser 3 ofoptical cavity length 12 emitting light optically coupled to theresonant optical cavity 2, wherein the external cavity laser comprise asemiconductor distributed feedback laser 7 as gain media, output coupler8, and another mirror 20 forming a laser cavity; mode matching optics 10configured to couple the laser light to resonant cavity 2 via cavitycoupling mirror 5. System 1 also comprises detector system 4 formeasuring absorption of laser light by the gas in resonant opticalcavity 2, detector system 4 being operable to transmit a data signalcorresponding the absorption of laser light by the gas in resonantoptical cavity 2, detector system 4 separated from mirror 6 by opticallength 13. Continuous-wave stepwise tunable external cavity laser 3 isoptically isolated from cavity 2 by optical isolator 9. Continuous-wavestepwise tunable external cavity laser 3 is fiber coupled to opticalfiber 15. Mode matching optics 10, which can be a source of scatteredlight couple back to cavity 2 is separated from coupling mirror 5 byoptical length 14.

FIG. 3 is another schematic diagram of a system 1 of the presentinvention. In FIG. 3, light beams are illustrated by solid lines. Forclarity of presentation, various standard elements such as lenses andmirrors used for focusing and directing beams are not described; suchelements are well known in the art. Processor and electrical connectionsare not shown. System 1 comprises resonant optical cavity 2 of opticalcavity length 11 containing a gas and having at least two cavitymirrors, one of which is cavity coupling mirror 5, another mirror 6 isused to transmit the light measured by the photo-detector. System 1 alsocomprises continuous-wave stepwise tunable external cavity laser 3 ofoptical cavity length 12 emitting light optically coupled to theresonant optical cavity 2, wherein the external cavity laser comprise sasemiconductor distributed feedback laser 7 as gain media and outputcoupler forming a laser cavity; mode matching optics 10 configured tocouple the laser light to resonant cavity 2 via cavity coupling mirror5. System 1 also comprises detector system 4 for measuring absorption oflaser light by the gas in resonant optical cavity 2, detector system 4being operable to transmit a data signal corresponding the absorption oflaser light by the gas in resonant optical cavity 2, detector system 4separated from mirror 6 by optical length 13. Continuous-wave stepwisetunable external cavity laser 3 is optically isolated from cavity 2 byoptical isolator 9. Continuous-wave stepwise tunable external cavitylaser 3 is fiber coupled to optical fiber 15. Mode matching optics 10,which can be a source of scattered light couple back to cavity 2 isseparated from coupling mirror 5 by optical length 14. The fiber coupledoptical amplifier or optical modulator 16 is placed betweencontinuous-wave stepwise tunable external cavity laser 3 and resonantoptical cavity 2.

FIG. 4 is a schematic diagram of a system 1 of the present invention. InFIG. 4, light beams are illustrated by solid lines. For clarity ofpresentation, various standard elements such as lenses and mirrors usedfor focusing and directing beams are not described; such elements arewell known in the art. Processor and electrical connections are notshown. System 1 comprises optical cell 17 containing a gas and having atleast two cell windows, one of which is cell coupling mirror 18, anotherwindow 19 is used to transmit the light measured by the photo-detector.System 1 also comprises continuous-wave stepwise tunable external cavitylaser 3 emitting light optically coupled to optical cell 17, wherein theexternal cavity laser comprise semiconductor distributed feedback laser7 as gain media and output coupler 8 forming a laser cavity; modematching optics 10 configured to transmit the laser light to opticalcell 17 via cell window 18. System 1 also comprises system 4 formeasuring the intensity of the laser beam after the laser beam passesthrough the gas in the optical cell 12, detector system 4 being operableto transmit a data signal corresponding the absorption of laser light bythe gas in optical cell 17. Continuous-wave stepwise tunable externalcavity laser 3 is optically isolated from optical cell 17 by opticalisolator 9. Continuous-wave stepwise tunable external cavity laser 3 isfiber coupled to optical fiber 15. Mode matching optics 10 is used toform a laser beam entering optical cell 17.

FIG. 5 illustrates how a laser tuning curve may depend on the opticalfeedback to the gain media in an external laser cavity, wherein themedia is a DFB laser. The feedback strength is increasing from FIG. 5Ato FIG. 5D. FIG. 5A illustrates the laser tuning curve of a free runningDFB laser with no feedback from any external to the DFB laser sources.FIG. 5B illustrates an effect of a weak feedback when smooth waves areappeared on the tuning curve. FIG. 5C shows an effect of an intermediatefeedback when sharp transitions are appeared on the tuning curve. In theintermediate feedback regime steps are almost equidistant in thefrequency space and correspond to the external cavity free spectralrange. FIG. 5D shows an effect of a strong feedback when steps on thetuning curve become irregular and differences of frequencies of twoadjusted steps exceed the external cavity free spectral range. The lasertuning curve of a DFB laser with no feedback from any external to theDFB laser sources are shown in FIG. 5B, FIG. 5C, and FIG. 5D by dashlines. Due to the optical feedback steps 30 are formed how the lasertuning curve.

The invention claimed is:
 1. An optical absorption spectroscopy basedgas analyzer system for detecting one or more chemical species in agaseous medium, the system comprising: an optical cell containing a gasmedium having a chemical species to be measured; a continuous-wave laseremitting laser light coupled to the cell, wherein the laser is capableof emitting a predominately single frequency light at a first wavelengthand the laser is capable of emitting a predominately single frequencylight at a second wavelength, and wherein the laser cannot becontinuously tuned from the first wavelength to the second wavelength; adetector system for measuring the intensity of the laser lighttransmitted through the optical cell and generating a data signal; awavelength meter system capable of measuring wavelength the laser lightbetween the first wavelength and the second wavelength, determining anevent of a laser tuning discontinuity, while the laser is tuned from thefirst wavelength to the second wavelength, and generating a laserwavelength data signal; and a processor capable of conducting theabsorption spectroscopy analysis of the gaseous medium based on the datasignal from the detector system and the laser wavelength data signalfrom the wavelength meter system received while the laser was tuned fromthe first wavelength to the second wavelength.
 2. The system of claim 1,wherein the laser selected from the group consisting of: external cavitylaser, distributed feedback laser, distributed Bragg reflector laser,volume holographic grating laser, discrete mode laser.
 3. The system ofclaim 1, wherein the laser selected from the group consisting of: diodelaser, quantum cascade laser, interband cascade laser, fiber laser. 4.The system of claim 1 further comprising a means for optical isolationof the laser from the optical cell.
 5. The system of claim 1, whereinthe optical cell is a multi-pass spectroscopic absorption cellcomprising two or more optical mirrors.
 6. The optical mirror of claim5, wherein an optical mirror substrate made from a light absorbingglass.
 7. The optical mirror of claim 5, wherein a surface roughness ofa surface of a mirror substrate coated with a reflective coating is lessthan 50 Angstrom.
 8. The system of claim 5, wherein the multi-passspectroscopic absorption cell further comprising an absorption maskmountable in the internal space of the cell, wherein a mask wall aplurality of apertures is formed, in which the positions of someapertures are adapted to a propagation path of laser light inside themulti-pass spectroscopic absorption cell.
 9. The system of claim 1,wherein the optical cell is used in the integrated cavity outputspectroscopy (ICOS) configuration.
 10. The system of claim 1 furthercomprising a means for changing an optical path length of the laserlight in the optical cell.
 11. The system of claim 1 further comprisinga pressure sensor for measuring pressure of the gaseous medium in theoptical cell.
 12. The system of claim 1 further comprising a temperaturesensor for measuring temperature of the optical cell.
 13. The system ofclaim 1 further comprising a pressure control element configured tocontrol pressure of the gaseous medium in the optical cell.
 14. Thesystem of claim 1 further comprising a temperature control elementconfigured to control temperature of the optical cell.
 15. The system ofclaim 1 further comprising a gas multiplexer capable deliver one or moregases to the optical cell.
 16. The system of claim 1 further comprisingat least one of the following means capable of changing a concentrationof a chemical species in the gaseous medium: a cold trap, selectivelypermeable membrane, a catalytic oxidizer, a gas absorber, a gashumidifier.
 17. The system of claim 1, wherein the system furthercomprising a detector for measuring an intensity of the laser lightemitted by the laser.
 18. The system of claim 1, wherein the systemfurther comprising a detector for measuring an intensity of a lightpropagated from the optical cell toward the laser.
 19. The system ofclaim 1, wherein the system further comprising a wavelength monitor formeasuring a wavelength of the laser light emitted by the laser.
 20. Thesystem of claim 1, wherein the system further comprising a gas referencecell for calibrating laser light frequencies.